The Bottle-Nosed Porpoise

Abstract

The order Cetacea comprises more than 40 species of marine mammals that live their long lives in the waters of the great oceans or, much less frequently, in rivers (the Amazon) or estuaries. All adult Cetacea are large, and one, the great blue whale, is the largest animal that ever lived—larger than the largest dinosaur. The great blue whale is almost as long as a football field and weighs up to 150 tons (136 metric tons). It is presumed that Cetacea developed from primitive land carnivores, but their origin and return to the sea lack fossil definition. In the Eocene Age, Cetacea were already abundant. They have large torpedolike bodies with forelimbs serving as flippers, no hindlimbs, a dorsal fin, and notched, flattened tails called “flukes” that are powerful aids to swimming and diving. Over time, their nostrils have moved to the top of the head as single or double blowholes. When whales exhale, a cloud of water drops and vapor is blown out. These blow clouds form a unique pattern that differs from species to species. Ordinarily, Cetacea breathe every 2–3 min, but they can dive to great depths for food and stay under water for up to 40 min without breathing. The porpoises and about half the whales have teeth and are carnivorous, eating only flesh, mostly fish or squid. The other whales have no teeth but rows of baleen (whalebone) that strain krill, the plankton and larval mixture that constitutes their principal food. The gestation period in Cetacea is almost a year. The young are then nursed for another year. Their life expectancy is long, 25–40 years, so that each female may produce 10–15 offspring in a lifetime.